The present invention relates to motor drives and in particular to an improved inverter used in such motor drives.
Motor drives are used to control the speed, torque, or other operating characteristics of AC induction motors.
In a typical large motor drive, three-phase power from a power line is rectified and filtered to provide a source of DC power. The DC power is provided to an inverter which converts the DC power back again to synthesized AC power that is used to drive the motor. By changing the frequency of the synthesized AC power, the speed or torque of the induction motor may be affected.
In operation, a controller within the motor drive receives a command signal from a user, for example, torque or speed, and provides the inverter with input signals indicating the desired characteristics of the synthesized AC power needed to achieve that torque or speed. The inverter receives the input signals and converts them to gate pulses driving solid state semiconductor switches, such as insulated gate bipolar transistors (IGBT), rapidly between on and off states in a class D or switching mode. The result is a duty cycle or pulse width modulated pulse train whose average voltage or current mirrors that of the desired synthesized AC power. Such switched operation is power efficient because it runs the solid state switching devices principally in a low power dissipation region where the solid-state devices are either fully conductive or non-conductive.
Despite the efficiency of such inverters, the voltage output of such inverters may not be an accurate representation of the input inverter signals particularly at lower voltages. Much of this accuracy problem appears to result from a non-linearity of the characteristics of the switching devices. For example, a variable delay in switching speed in the devices will affect the amplitude and phase of the synthesized waveform.
One method of addressing inverter voltage inaccuracy is by modeling the nonlinearity of the switching devices and building an inverse model into the circuitry that generates the gate pulses for the switching devices. To the extent that such nonlinearities may vary from switching device to switching device, this approach requires a cumbersome adjustment of the model used in each inverter.